<template>
  <div class="list">
    <ul class="item"
    v-for="item of letters" :ref="item" @touchend="handleTouchEnd" @touchmove = "handleTouchMove" @touchstart.prevent = "handleTouchStart" @click="handleLetterClick" :key="item">{{item}}
    </ul>
  </div>
</template>

<script>
export default{
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  computed: {
    letters () {
      const letters = []
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: true
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change', e.target.innerText)
    },
    handleTouchStart (e) {
      this.touchStatus = true
    },
    // 节流限制函数执行频率
    handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.timer) {
          this.timer = false
          setTimeout(() => {
            const touchY = e.touches[0].clientY - 79
            const index = Math.floor((touchY - this.startY) / 20)
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index])
            }
            this.timer = true
          }, 16)
        }
      }
    },
    handleTouchEnd (e) {
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/varibles.styl'
  .list
    display flex
    flex-direction column
    justify-content center
    position absolute
    top 1.58rem
    right 0
    bottom 0
    width .4rem
    color $bgColor
    .item
      text-align center
      line-height .4rem

</style>
